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How an LM339 works

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QiQ

New Member
I need to trig a relay based on an adjustable voltage threshold (if the signal goes say below 2.8V, I want the relay to trig open). I purchased LM339s (quads) to do so with a common voltage reference for all comparators.

Questions:

1) Vcc specs seem to be 36V or +/-18... is that the max allowed supply or the REQUIRED supply? (I'm working with 5V CMOS everywhere else on the board and don't have access to a 36V source)

2) I would like the output to be latched until reset. Can I add a simple IC at the outputs to achieve that?
 

QiQ

New Member
I got my answer as far as the power supply goes: it does work with 5V. Now for the latched outputs (possibly driving LEDS)...
 

BrownOut

Banned
There are lots of ways to latch the output. Post a schematic of your intended circuit, and someone will probably know the best way to do it.
 
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QiQ

New Member
There are lots of ways to latch the output. Post a schematic of your intended circuit, and someone will probably know the best way to do it.
I was looking for an IC that intergrated several latches... I've got 8 outputs to latch with limited space to fit the circuitry.
 

BrownOut

Banned
That's exactly why I asked the OP to post a schematic. Without that, there isn't any way to know if it'll work or not. BTW, it's one enable per 4-bit latch.

EDIT: My effort to help a poster is proportional to his effort to help me understand his proposal/objective.
 
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Roff

Well-Known Member
The ideal part in my mind would be an octal D FF with common D inputs and individual clocks - just the opposite of available octal D's.
Of available parts, 74xx279 or CD4043/4044 are the best I can find. They are quad SR latches. Each comparator output would have to be coupled to one of the latch inputs through an RC differentiator (or a digital differentiator).
 

QiQ

New Member
That's exactly why I asked the OP to post a schematic. Without that, there isn't any way to know if it'll work or not. BTW, it's one enable per 4-bit latch.

EDIT: My effort to help a poster is proportional to his effort to help me understand his proposal/objective.
I'm sorry if I dissapointed you. I like to be guided, try a few things on my own following advices, succed/fail and give feedback based on that.

The circuit has to perform the opening of a relay if one of its inputs goes over a certain voltage. There are 4 devices running in series monitored so that if any of them has its temperature above a certain point, it trigs the relay. The inputs are fed directly to the LM339 (which after verification, was part of the problem: it required its outputs pulled up with 3k resistors). The 4 outputs of the LM339 are fed into a 4 input NAND gate, whose output drives the relay (through a transistor).

I guess there may have been a better way of performing the same task for someone more experience, but I had very little time to make it work. So far it does on the breadboard, we'll see when everything is soldered.

PS: the reason why I wanted latches was to eliminate oscillation around the threshold and to drive 4 leds to know which one of the deviced trigged the over temp.
 
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BrownOut

Banned
Latching isn't required to eliminate oscillation around the threshold. All you need is hysterisis in your comparator. It's much simplier. BTW, I understand the SN74100 is obsolete. If you want latches, just take Roff's suggestion and use D-tpe ff's.
 

QiQ

New Member
Latching isn't required to eliminate oscillation around the threshold. All you need is hysterisis in your comparator. It's much simplier. BTW, I understand the SN74100 is obsolete. If you want latches, just take Roff's suggestion and use D-tpe ff's.
Latching was primarely required to light the corresponding led. Its second purpose, while doing so, was to play the hysteresis role.

Thanks all.
 
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